Reuse is a big no-no. One of the worst possible ways to attempt to save $3.

You can use a plastic dollar store turkey baster to remove excess fluid from the reservoir.
Great 'specialty tool'!
That, or Williams-Sonoma sells stainless-steel basters with an MB logo on them...
(Do not use it for cooking afterwards, just to be safe...)

Some folks insist that one should never compress the pistons without the bleeders open to prevent the most contaminated fluid that lives in the calipers from getting pushed up through the Master.

I only take this extra precaution if the fluid has not been flushed for far too long.